This invention relates to an exposure method and an exposure apparatus, and, more particularly, it relates to exposure method and apparatus for the manufacture of semiconductor circuit devices.
Recent development in semiconductor technology has resulted in higher capacities and further miniaturization of semiconductor circuit devices. Along with such trend, photolithography techniques such as an optical exposure process have become more and more dominant with the development of high resolution lenses. In such exposure systems, a short wavelength of light within the deep UV range has recently been used to transfer and print a circuit pattern of a mask or reticle onto a wafer. This is because the resolution for the minimum line width of the circuit pattern to be printed on the wafer is proportional to the wavelength of the light.
Conventionally, heavy hydrogen lamps or Xe-Hg lamps have been used as the deep UV light sources. These lamps provide continuous emission in both cases of DC energization and AC energization. In view of such feature, the amount of exposure for the wafer has been achieved by analog-like control systems such as, for example, a timer control system for controlling the exposure time by means of a timer, or an integrating exposure-meter system in which the amount of exposure is integrated and the exposure is continued until the integrated exposure reaches a predetermined value.
Use of such light sources, however, involves inconveniences, because only a decreased output is obtainable in the deep UV range and the sensitivity of the photoresist material applied to the wafer surface is low. This results in a longer exposure time and a decreased throughput.
It has recently been found that a high-power and high-luminance laser oscillatingly emitting a beam in a short wavelength region (deep UV region) is effective to be applied to the exposure apparatus. An example of such laser is an excimer laser. The excimer laser intermittently emits a laser beam at the repetition-frequency of approx. 200-300 Hz. The emission time (the duration of each pulse) is about 10-30 nsec. Because of the high power of the excimer laser, only one pulse emission is usually required to provide a sufficient amount of exposure relative to an ordinary resist material. Therefore, the exposure time of the order of 0.01 sec. is attainable.
However, the pulse energy of the excimer laser contains fluctuation of about .+-.5%. This pulse energy fluctuation directly leads to irregularity in the exposure energy to be applied to chip areas on the wafer. The irregular exposure produces serious effects on the resolving power and the reproducibility of line width after development of the resist material and, therefore, decreases yield of chips. However, the excimer laser is a pulse-oscillation type laser, as compared with the conventional heavy hydrogen lamps or Xe-Hg lamps. For this reason, the above-described analog-like control system for controlling the amount of exposure could not be used, as it is, in exposure apparatuses employing excimer lasers.